Tail gas streams originate from various sources. They are found in refinery off-gases as well as Claus units that are unable to convert all gaseous sulfur species to elemental sulfur. These tail gases contain SO2 and H2S at levels exceeding permissible emission limits which are currently set at 10 ppm H2S and 250 ppm SO2 in the United States. Tail gas compositions vary widely depending on the application. Often steam, syngas, and/or CO2 are found in tail gases. Tail gases are mostly free of O2 but often contain low levels of H2.
One way to treat tail gases is by hydrotreating and amine scrubbing. Hydrotreating requires the whole gas stream to be heated to reaction temperature following a gas cool-down from 400° C. to near ambient temperatures prior to use. Inherent in this process is a significant energy penalty due to the heating and cooling steps required. The amine regeneration produces concentrated H2S which is returned to a Claus unit where it is converted to elemental sulfur.
Alternatively, the gas can be oxidized in a burner to form SO2 as the only sulfur species. This option also requires a cool-down phase and additional equipment to scrub the SO2 and to regenerate the scrubbing material. This is known as the CANSOLV® process (CANSOLV is a registered trademark of Cansolv Technologies, Inc.) and the regeneration produces concentrated SO2 which is recycled to a Claus unit.
Claus units are useful for sulfur removal. However, Claus units typically operate at low temperatures (around 280° C.) requiring large waste heat boilers to cool feed gases and achieve high conversions only through multiple reactor beds thereby requiring a larger energy footprint. Further, their maximum sulfur conversion is 92% when current environmental regulations require greater than 98.5% sulfur recovery.
Accordingly, a need exists for a process to remove contaminants from a tail gas stream that eliminates the heating-up and cooling-down steps from the alternative processes. Additionally, a need exists for a sulfur recovery process which is economical and would require a smaller energy footprint.